1. Field of the Invention
This invention relates to a method of manufacturing a pipe-shaped product with end elements welded to the ends thereof, such as a push rod for operating intake valves and exhaust valves of internal combustion engines.
2. Description of the Prior Art
Hitherto, a pipe-shaped product with end elements, such as a push rod used to operate intake valves and exhaust valves of internal combustion engines, was manufactured by pressing a bottom surface of a carburized and hardened end element having a concave end surface against one tapered annular end surface of a pipe chucked by a chuck by means of an electrode provided at the point of a piston rod of an air cylinder, heating and fusing them by means of large current to perform projection-welding, and similarly projection-welding a bottom surface of another carburized and hardened end element having a convex end surface to the other tapered annular end surface of the pipe.
According to the foregoing conventional method, as shown in FIG. 6A, if an end element 3 is pressed against a tapered annular end surface 2 of a pipe 1 with some inclination because of looseness of a piston rod of an air cylinder or wear of a chuck for a pipe, the tapered annular end surface 2 of the pipe 1 abuts partially on the bottom surface of the end element 3; as a result, an abutting portion of the annular end surface is crushed and deformed. In this case, since the deformation of the tapered annular end surface 2 occurs only radially, the deformation resistance of the annular end surface to crushing increases slightly, so that deformation advances in response to subsequent pressing force. Therefore, the initial inclination can hardly be avoided even by the deformation resistance; thus, such a degree of inclination is retained even after the completion of pressing (see FIG. 6B). Consequently, the width of a welding surface portion at the point of the pipe 1 that is formed by pressing the end element 3 thereagainst becomes non-uniform in the circumferential direction as shown in FIG. 7, or the ratio of largest width to smallest width becomes as large as three.
Since this type of push rod is used in a drive section, including intake valves and exhaust valves, of an internal combustion engine, it is required to have sufficient tensile strength, shear strength and fatigue strength, the fatigue strength must be uniform especially in the circumferential direction, and therefore, the pipe 1 and the end element 3 must be welded together after the welding surface portion is rendered as uniform as possible.
However, where the welding surface portion is non-uniform in the circumferential direction as described above, since the value of current per unit area becomes substantially certain even when welding is performed under the same conditions of voltage, current, welding time, etc., the value of welding current varies largely from portion to portion in the circumferential direction. Consequently, the width of welding, welded condition between the end element and the pipe, extent of influence of heat, strength of welding, and the like become non-uniform in the circumferential direction, with the result that the tensile strength, shear strength, and fatigue strength take small values; thus, no sufficient strength and durability cannot be obtained. Accordingly, in the conventional method, the end element 3 must be accurately set and welded to the tapered annular end surface 2 of the pipe 1 without any inclination, and for uniform welding, high skill is demanded.